Ink supply system for a jet ink printer

ABSTRACT

There is disclosed an ink recirculation and replenishment system for use in combination with an ink jet printer. Ink which is formed into drops but not deposited on the print receiving member is collected in a vacuum tank and thereafter recirculated by a reciprocating pump at a steady volumetric rate. There is a pressure tank for regulating pressure of the pumped ink and returning excess ink directly to the pump. Means are provided for supplying an ink replenishment liquid to the system as well as fresh ink in volumetric amounts as required to replace fluid lost by printing and by evaporation. The replenishment liquid contains a solvent, which in the case of aqueous base ink will be water, and other agents for preventing deterioration of the recirculated ink. A depletion sensor, preferably a conductivity probe, is provided for automatic control of replenishment liquid addition.

iliiited States Patent [1 1 I Ielgeson et a1.

[ INK SUPPLY SYSTEM FOR A JET INK PRINTER [75] Inventors: Gaylord A. l-lelgeson; Lysle D. 57

Cahill, Dayton, Ohio 1 [73] Assignee: The Mead Corporation, Dayton,

Ohio

[22] Filed: Oct. 24, 1972 [21] Appl. No.: 299,900

[52] 11.8. CI 346/75, 137/5, 137/93, 346/140 [51] Int. Cl. Gllld 15/18 [58] Field of Search 346/75, 140; 137/5, 137/93 [56] References Cited UNITED STATES PATENTS 3,361,150 l/1968 Homer 137/93 3,512,173 5/1970 Damouth 346/75 [451 Sept. 25, 1973 Primary Examiner-Joseph W. Hartary Attorney-Lawrence B. Biebel et al.

ABSTRACT There is disclosed an ink recirculation and replenishment system for use in combination with an ink jet printer. Ink which is formed into drops but not deposited on the print receiving member is collected in a vacuum tank and thereafter recirculated by a reciprocating pump at a steady volumetric rate. There is a pressure tank for regulating pressure of the pumped ink and returning excess ink directly to the pump. Means are provided for supplying an ink replenishment liquid to the system as well as fresh ink in volumetric amounts as required to replace fluid lost by printing and by evaporation. The replenishment liquid contains a solvent, which in the case of aqueous base ink will be water, and other agents for preventing deterioration of the recirculated ink. A depletion sensor, preferably a conductivity probe, is provided for automatic control of replenishment liquid addition.

16 Claims, 1 Drawing Figure so 581 Fr 62 g PRESSURE SUPPLY Patented Sept. 25, 1973 wmzwwmmm m2 INK SUPPLY SYSTEM FOR A JET INK PRINTER BACKGROUND OF THE INVENTION This invention relates to jet drop printing devices, and particularly to the inking systems thereof. A typical construction and arrangement of such a jet drop printing device is disclosed in U.S. Pat. No, 3,588,906, issued June 28, I971, and assigned to the assignee of this application.

In the operation of such printing devices a suitable conductive ink is directed through a drop generator for formation into a stream of uniformly sized and regularly spaced drops. These drops are selectively charged in accordance with an input intelligence signal and thereafter are either deposited on a print receiving member or caught and collected by a suitable catcher. However, for many printing applications the ratio of printed to unprinted area is quite small so that most of the ink passing through the drop generator is collected by the catcher. Accordingly, it has become desirable to achieve economy of operation by recirculating and reusing the collected ink.

During recirculation and reuse of suchink it has been discovered that the used ink differs from fresh ink in specific gravity, viscosity, and conductivity. For instance in a study of aqueous base inks of the type disclosed in copending U.S. application Ser. No. 153,426, filed June 15, 1971, it was found that between and percent of the solvent was being lost during the drop forming and catching process. The exact quantity of solvent lost is dependent upon the vapor pressure of the solvent and hence varies with the drop temperature, the relative humidity of the air stream along the drop path or trajectory, the time duration of drop exposure, and the rate of air flow through the system.

It has been observed that any loss of solvent increases the ionic strength of the additives, particularly the electrolyte, in such ink, up to the point of saturation. Above this point precipitation of the dye stuff and other ink constituents occurs, causing clogging of the fine pores of the filter in the ink recirculating system, and also causing clogging of the small jet orifices.

Observation also indicated that not only the solvent, but other constituents of the ink were being lost from the system. In the case of the particular ink disclosed in said copending application Ser. No. 153,426, the loss of the metal chelating agent glucono-8-lactone was observed to occur. The symptoms that led to discovery of loss of this component were clogging of the jets and the filters. Apparently the metal ion complexing agent, the glucono-S-lactone, may have complexed in the process of circulation through various stainless steel components of the ink conducting system.

SUMMARY OF THE INVENTION In accordance with this invention it has been found that deterioration of jet drop printing ink may be avoided by adding an appropriate replenishment solution in a novel manner as hereinafter described. In the case of the ink described in Ser. No. 153,426 an appropriate replenishment solution has been found to comprise distilled water; glucono-S-lactone, a corrosion inhibiting agent; I, 2,-6 hexanetriol, an anti caking agent; and an anti-microbial compound manufactured by Dow Chemical Company under the name Dowicil 100. The anti-microbial compound which is known chemically as l-( 3-chloroallyl )-3,5,7-triazal azoniaadamantane chloride inhibits the growth of Penicillium sp.

The replenishment liquid is packaged in a plastic bag within a stiff cubic container and placed spout down, above a mixing valve. A supply of fresh ink is similarly placed above the mixing valve, and the output from the mixing valve is used to make up system volumetric losses. This output together with recirculated ink is pumped toward the jet drop printing head by a reciprocating pump. The pump moves ink at a steady volumetric rate and creates a back pressure in the pump supply lines. The output from the mixing valve passes, through a pressure operated check valve enroute to the pump so that recirculated ink is preferentially pumped; fresh ink or replenishment liquid being pumped only as required to make up volumetric losses in the system. A depletion sensor, preferably a conductivity sensor may be placed at the output side of the pump and used to control the position of the mixing valve. The pressure of the ink at the output side of the pump is regulated by a novel pressure tank for delivery to the printing head. As a result there is provided a simple reliable ink recirculation and replenishment system requiring very few electrical controls.

Accordingly it is an object of this invention to eliminate deterioration of ink in a recirculating ink jet printer by adding controlled amounts of replenishing liquid thereto.

It is another object of this invention to make up volumetric losses of ink in a recirculating ink jet printer by adding controlled amounts of fresh ink thereto.

It is still another object of this invention to provide simple improved apparatus for recirculating ink in an ink jet printing system.

Another object of the invention is to provide improved pressure regulating means for an ink supply system for an ink jet printer.

Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS The FIGURE is a schematic diagram including fluidic and electrical controls for an ink recirculation and replenishment system for an ink jet printer.

DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the invention is illustrated in the FIGURE wherein an ink drop generator 10 generates a series of drops 18, some of which are deposited on a print receiving member 19 and some of which are intercepted by a catcher 24. The ink which is caught by catcher 24 is recirculated to drop generator 10 by passing through an ink collection receptacle 1 I, then through a withdrawing means such as a reciprocating pump 12, and then through a pressurizing means such as pressure tank 13. There is an overflow line 14 from pressure tank I3 which delivers excess ink to ink supply line 15 which is the primary supply line for pump 12. Pressurized ink for use by drop generator 10 leaves pressure tank 13 by an exit line 47 which leads to a filter 48 and a surge tank 49.

Drop generator 10 comprises an orifice assembly 16, a stimulator 17, a charging electrode 20, a charge signal amplifier 21 and a pair of deflection electrodes 22 and 23. Drop formation charging and deflection, which forms no part of this invention, is carried out as described for instance in Van Brimer et al U.S. Pat. No. 3,588,906.

lnk collection receptacle 11 is a vacuum tank which comprises a vessel 25 and a float valve assembly 26. Vacuum within vacuum tank 1 1 is created by a vacuum pump 29 and is regulated by a regulator 30 which may have a filter 31 attached thereto. Float valve assembly 26 causes incoming ink to accumulate in vacuum tank 11 until a predetermined volume has been collected. Thereafter float valve assembly 26 rises and permits collected ink to flow into exit line 27 and thence through check valve 28 and supply line to pump 12. Pump 12 pumps liquid in steady constant volume pulses, thereby creating pulses of reduced pressure in supply line 15 and at the operating control for check valve 28. Thus collected ink is delivered from vacuum tank 1 l to pump 12 only when a predetermined volume of ink has been collected in vessel 25 and then only if the pressure across check valve 28 exceeds a predetermined minimum, typically about psid. There is also a check valve 68 for supplying line 15 which may be set for opening upon sensing of pressure differential of about 5 psid. Thus if vacuum tank 11 is low on collected ink and float valve 26 is closed, pump 12 will draw liquid for pumping through check valve 68 which provides fresh ink or a replenishing liquid as hereinafter described. However, check valves 28 and 68 are relatively set so that pump 12 will draw liquid preferentially from vacuum tank 11.

Pump 12 comprises a flexible diaphragm 32, a hydraulic liquid pool 33 and a reciprocating shaft 34. Forward motion of shaft 34 extends diaphragm 32, closes an inlet valve 35, opens an exit valve 36, and pumps a controlled volume of ink toward pressure tank 13. Return movement of shaft 34 causes relaxation of diaphragm 32, closing of exit valve 36, opening of entrance valve 35, and withdrawal of a fixed volume of fluid from line 14, check valve 28, and/or check valve 68. Reciprocation of shaft 34 is produced by a motor 37, a cam 38, and other linkage as illustrated.

There is a surge tank 39 connected to the exit side of pump 12 for damping out pressure surges in the pumped ink. The pumped ink bypasses surge tank 39 as illustrated, and does not pass therethrough.

After leaving pump 12 the pumped ink passes through a replenishment sensing cell 40, and thence through filter 41 enroute to pressure tank 13. Replenishment sensing cell 40 may comprise a pair of conductive probes 69 and 70 and a thermistor 74. Cell 40 is configured for measurement of the conductivity of the ink flowing therethrough, and accordingly has a driver 71 and an amplifier 72 connected respectively to probes 69 and 70. Driver 71 generates an electrical signal which is transmitted to amplifier 72 via the ink in cell 40. The amplitude of the signal generated by the amplifier 72 is thus a function of the conductivity of the fluid in cell 40. The output from amplifier 72 is fed to a trigger circuit 73 which fires when the conductivity of the ink in cell 40 is above some predetermined level.

When trigger 73 fires, it activates solenoid 67 to position valve 66 for admission of fluid from tank 64 to line 65. However, in the absence of a trigger-signal, valve 66 is ordinarily positioned for admission of fluid from tank 63 into line 65. Tank 63 contains fresh ink, while tank 64 contains a replenishing solution as hereinafter described. Thus it is seen that when pump 12 reciprocates on the backward stroke, it creates a back pressure at the pump inlet which causes pumping first of overflow from line 14, then collected ink from vacuum tank 11, and finally fresh ink from tank 63 in volumetric amounts as required to augment overflow ink and collected ink. However, if the conductivity as measured by cell 40 is above a predetermined level, then replenishing fluid replaces fresh ink as a volumetric supplement for input to pump 12.

Conductivity variations due to temperature changes in the recirculated ink are compensated by a temperature compensating network which is connected to thermistor 74. Temperature variations within the ink in cell 40 cause resistance changes in thermistor 74 and these changes are sensed by the network 75. Network 75 generates a compensating signal which is added to the output signal from driver 71 for application to probe 69.

After leaving cell 40, the ink flows through a filter 41, as stated above, and thence through a line 46 into pressure tank 13. Pressure tank 13 comprises a pressurizing vessel 42, a standpipe 43, a diaphragm 44 and an air cavity 45. Air is supplied to cavity 45 at a regulated pressure for urging diaphragm 44 downwardly against the entrance to standpipe 43. Ink normally enters pressure tank 13 via entrance line 46 and leaves via exit line 47, but the input rate is preferably greater than the output rate. Consequently the incoming ink fills vessel 42 and bears upwardly against diaphragm 44. When the pressure of the ink within vessel 42 exceeds the pressure of the air in-cavity 45, the ink begins to enter standpipe 43 at the top thereof and flows downwardly to overflow line 14. Thus the volume and pressure of the ink delivered into line 47 for use by drop generator 10 is carefully regulated.

The ink which leaves pressure tank 13 has been observed in some cases to exhibit small pressure surges which would disturb the operation of drop generator 10. Thus the ink is directed through a surge tank 49 which comprises a surge cell 52 and input and output collections thereto. The ink enters surge cell 52 wherein it is met by an entrapped pocket of air 53 which absorbs minor pressure variations therein. Thereafter the ink flows from surge cell 52 to the orifice assembly 16 for formation into drops 18 as aforesaid.

Pressurized air for use by pressure tank 13 is supplied by an external high pressure air supply. The pressurized air passes through a manually controlled inlet valve 62 thence through an air filter 61 and an air pressure regulator 60. There is a bleed tank 54 which is connected to filters 41 and 48 by lines 57 and 58 respectively. Bleed tank 54 bleeds off air which may become entrapped in filters 41 and 48. Bleed tank 54 is primarily important during start-up of the system, and regulating valves 55 and 56 regulate the operation thereof.

The recirculating and replenishment system may also comprise a series of guages 76, 77, 78 and 79, and a sampling valve as shown in FIG. 1. Typically pump 12 may pump fixed volumes of ink at a rate of about two pulses per second and is capable of pumping a pressure which varies from about 75 to psi. at the pump outlet. This pressure is regulated by pressure tank 13 so that the pressure rate at guage 78 is about 2 5 psi.

Containers 63 and 64 may be commercially available containers of the bag-in-box variety comprising a flexible plastic bag within a rigid container. Such containers are normally supplied with spouts so that they may be readily connected to the input lines for mixing valve 66 and thus supply liquid on demand from their inner plastic bags. The replenishing liquid supplied by container 64 may be tailored to meet the needs of the particular ink used by the system. For instance, in the case of a water base anti cockling ink as described in Ser. No. 153,426, an appropriate replenishing liquid has been found to comprise distilled water; about 0.2 percent by weight of 1, 2, 6-hexanetriol; about 0.05 percent by weight of glucono8-lactone; and about 0.5 grams per liter of an anti-microbial compound manufactured by Dow Chemical Company under the name Dowicil 100.

The l, 2, fi-hexanetriol is an agent which is added both to fresh ink and to the replenishing liquid for the purpose of slowing down evaporation and inhibiting the tendency of the ink to cake in the jet assembly. Glucono-S-lactone is also added to the fresh ink as well as to the replenisher for the purpose of preventing precipitation of ink constitutents by metal contaminants, with the concentration in fresh ink being about twice that in the replenisher. The anti-microbial solution which is known chemically as l-(3-chloroallyl)-3,5,7-triaza-lazoniaadamantane chloride inhibits the growth of Penicillium sp, an airborne sporific fungus which tends to form filamentary colonies within the replenisher and thus to clog filters 41 and 48 and orifice assembly 16.

When the water or other solvent in the ink evaporates, the direct result is an increase in viscosity and specific gravity. However, it has been found that the conductivity of the ink also increases with the loss of solvent, so that increases in solids concentrations may be observed indirectly by measuring the ink conductivity. Accordingly the addition of replenishing liquid is preferably controlled by solenoid 67 which in turn responds to measurements made by the conductivity sensor 40, all as described above. Typically trigger 73 will be set so that replenishing liquid is delivered to the system when the measured conductivity exceeds about 00390 mhos per cem. This corresponds to a specific gravity of about 1.080. The fresh ink has a nominal specific gravity of 1.070.

It will be appreciated that while as much as 10 to percent of the solvent may be lost by ink passing through drop generator 116, this loss over a short period of time represents only a small fraction of the total amount of solvent in the entire system. Thus it is not necessary that driver 71, amplifier '72, trigger 73, and the temperature compensating network 75 operate continuously. Accordingly the foregoing elements are conveniently packaged to make an analyzing unit M which may be time shared with other jet drop printing systems. Alternatively a single jet drop printing system may comprise four drop generators such as drop generator 16 for printing four different colors in relative registration. Such a printer may comprise four ink recirculation and replenishment systems such as the system of HG. l but with a single time shared analyzing unit m.

While the form of apparatus herein described constitutes a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention.

What is claimed is:

l. A recirculating ink supply system for a jet drop printer comprising:

1. an ink collection receptacle connected for collection of excess ink from said jet drop printing system,

2. a supply line connected for recirculating reception of collected ink from said ink collection receptacle,

3. means for supplying fresh ink to said supply line,

4. means for withdrawing the fluid content from said supply line at a controlled volume rate,

5. pressurizing means for pressurizing the fluid withdrawn as aforesaid from said supply line and delivering the pressurized fluid to said jet drop printer, and

6. means for mixing a depletion correcting replenishment fluid with the recirculated ink delivered as aforesaid to said jet drop printer.

2; Apparatus according to claim 1 further comprising means for sensing a depletion state of said recirculated ink and controlling said mixing means in response thereto.

3. Apparatus according to claim 2, said mixing means being connected to said supply line, and said sensing means being connected for sensing the depletion state of fluid at the output side of said withdrawing means.

4. Apparatus according to claim 2, said sensing means comprising a conductivity sensor.

5. Apparatus according to claim 1 further comprising means for controlling the flow of fresh ink into said supply line in accordance with the back pressure created therein by said withdrawing means.

6. Apparatus according to claim 5 said fresh ink supply means comprising a fresh ink supply vessel and a feed line connecting said vessel with said supply line, and said means for controlling the flow of fresh ink comprising a check valve in said feed line.

7. Apparatus according to claim 6, said mixing means comprising a replenishment fluid supply vessel connected to said feed line and a mixture control valve for selectively admitting fresh ink or replenishment fluid into said feed line.

8. Apparatus according to claim 7, said pressurizing means having an overflow line connected to said supply line.

9. Apparatus according to claim further comprising a check valve between said ink collection receptacle and said supply line, the cracking pressure for said valve being less than the cracking pressure for the check valve in said feed line whereby said withdrawing means selectively withdraws from said supply line; first the overflow ink from said pressurizing means, next the ink from said collection receptacle, and lastly fresh ink or replenishment fluid depending upon the position of said mixture control valve.

10. Apparatus according to claim '9 further comprising a conductivity sensor for sensing the conductivity of the fluid withdrawn from said supply line and a solenoid for receiving a sensing signal from said sensor and positioning said mixture control valve in response thereto.

lll. Apparatus according to claim 1 said pressurizing means comprising:

1. an ink pressurizing vessel,

2. input and output connections to said vessel,

3. a standpipe mounted within said vessel and connected for drainage to said supply line,

4. a diaphragm for covering the entrance to said standpipe, and

5. means for. urging said diaphragm against said standpipe whereby said input connection receives ink at the delivery rate of said withdrawing means, and said output connection delivers pressurized ink at a rate as required by said jet drop printer, with surplus ink exiting the ink pressurizing vessel via said standpipe.

12. Apparatus according to claim 11 said means for urging said diaphragm against said standpipe comprising a gas chamber and means for delivering a pressurized gas thereto.

13. Apparatus according to claim 12 wherein the pressurized fluid flowing from said pressurizing means to said jet drop printer passes through a surge tank comprising a surge cell for pooling said pressurized fluid under an entrapped pocket of air and input and output connections thereto.

14. In a system for marking with individual drops of ink, including a supply reservoir of aqueous base marking ink containing normally solid dissolved materials, the liquid portions of such ink being volatizable in use whereupon the proportion of solids to liquids changes sufficiently to cause a material change in the flow characteristics of the ink,

a drop generator arranged to receive liquid from said reservoir and to project a stream of discrete drops along one trajectory,

means for deflecting selected individual drops into anther trajectory,

catcher means arranged to intercept drops following one of the trajectories and permitting drops following the other trajectory to proceed toward a receiving surface; and

a return connection from said catcher means to said reservoir; the improvement comprising means sensing the relation of volatizable to nonvolatizable portions of the liquid in the system,

means controlled by said sensing means for replenishing volatizable components of the marking liquid which are lost in the liquid system,

means for combining the replenished marking liquid with fresh marking ink, means for circulating the combined fluid toward said drop generator at a controlled volume rate, and

means for returning to said circulating means a portion of said combined fluid in variable amount as required to maintain a regulated pressure in the fluid received by said drop generator.

15. A system as defined in claim 14 including a filter in said return connection to remove contaminants entering the open part of the liquid system between the drop generator and the catcher means.

16. A system as defined in claim 14 wherein said sensing means includes a conductivity sensor. 

1. A recirculating ink supply system for a jet drop printer comprising:
 1. an ink collection receptacle connected for collection of excess ink from said jet drop printing system,
 2. a supply line connected for recirculating reception of collected ink from said ink collection receptacle,
 3. means for supplying fresh ink to said supply line,
 4. means for withdrawing the fluid content from said supply line at a controlled volume rate,
 5. pressurizing means for pressurizing the fluid withdrawn as aforesaid from said supply line and delivering the pressurized fluid to said jet drop printer, and
 6. means for mixing a depletion correcting replenishment fluid with the recirculated ink delivered as aforesaid to said jet drop printer.
 2. Apparatus according to claim 1 further comprising means for sensing a depletion state of said recirculated ink and controlling said mixing means in response thereto.
 2. a supply line connected for recirculating reception of collected ink from said ink collection receptacle,
 2. input and output connections to said vessel,
 3. a standpipe mounted within said vessel and connected for drainage to said supply line,
 3. Apparatus according to claim 2, said mixing means being connected to said supply line, and said sensing means being connected for sensing the depletion state of fluid at the output side of said withdrawing means.
 3. means for supplying fresh ink to said supply line,
 4. means for withdrawing the fluid content from said supply line at a controlled volume rate,
 4. Apparatus according to claim 2, said sensing means comprising a conductivity sensor.
 4. a diaphragm for covering the entrance to said standpipe, and
 5. means for urging said diaphragm against said standpipe whereby said input connection receives ink at the delivery rate of said withdrawing means, and said output connection delivers pressurized ink at a rate as required by said jet drop printer, with surplus ink exiting the ink pressurizing vessel via said standpipe.
 5. Apparatus according to claim 1 further comprising means for controlling the flow of fresh iNk into said supply line in accordance with the back pressure created therein by said withdrawing means.
 5. pressurizing means for pressurizing the fluid withdrawn as aforesaid from said supply line and delivering the pressurized fluid to said jet drop printer, and
 6. means for mixing a depletion correcting replenishment fluid with the recirculated ink delivered as aforesaid to said jet drop printer.
 6. Apparatus according to claim 5 said fresh ink supply means comprising a fresh ink supply vessel and a feed line connecting said vessel with said supply line, and said means for controlling the flow of fresh ink comprising a check valve in said feed line.
 7. Apparatus according to claim 6, said mixing means comprising a replenishment fluid supply vessel connected to said feed line and a mixture control valve for selectively admitting fresh ink or replenishment fluid into said feed line.
 8. Apparatus according to claim 7, said pressurizing means having an overflow line connected to said supply line.
 9. Apparatus according to claim 8 further comprising a check valve between said ink collection receptacle and said supply line, the cracking pressure for said valve being less than the cracking pressure for the check valve in said feed line whereby said withdrawing means selectively withdraws from said supply line; first the overflow ink from said pressurizing means, next the ink from said collection receptacle, and lastly fresh ink or replenishment fluid depending upon the position of said mixture control valve.
 10. Apparatus according to claim 9 further comprising a conductivity sensor for sensing the conductivity of the fluid withdrawn from said supply line and a solenoid for receiving a sensing signal from said sensor and positioning said mixture control valve in response thereto.
 11. Apparatus according to claim 1 said pressurizing means comprising:
 12. Apparatus according to claim 11 said means for urging said diaphragm against said standpipe comprising a gas chamber and means for delivering a pressurized gas thereto.
 13. Apparatus according to claim 12 wherein the pressurized fluid flowing from said pressurizing means to said jet drop printer passes through a surge tank comprising a surge cell for pooling said pressurized fluid under an entrapped pocket of air and input and output connections thereto.
 14. In a system for marking with individual drops of ink, including a supply reservoir of aqueous base marking ink containing normally solid dissolved materials, the liquid portions of such ink being volatizable in use whereupon the proportion of solids to liquids changes sufficiently to cause a material change in the flow characteristics of the ink, a drop generator arranged to receive liquid from said reservoir and to project a stream of discrete drops along one trajectory, means for deflecting selected individual drops into anther trajectory, catcher means arranged to intercept drops following one of the trajectories and permitting drops following the other trajectory to proceed toward a receiving surface; and a return connection from said catcher means to said reservoir; the improvement comprising means sensing the relation of volatizable to nonvolatizable portions of the liquid in the system, means controlled by said sensing means for replenishing volatizable components of the marking liquid which are lost in the liquid system, means for combining the replenished marking liquid with fresh marking ink, means for circulating the combined fluid toward said drop generator at a controlled volume rate, and means for returning to said circulating means a portion of said combined fluid in variable amount as Required to maintain a regulated pressure in the fluid received by said drop generator.
 15. A system as defined in claim 14 including a filter in said return connection to remove contaminants entering the open part of the liquid system between the drop generator and the catcher means.
 16. A system as defined in claim 14 wherein said sensing means includes a conductivity sensor. 